With advancing age vascular endothelial cells are more susceptible to injury and are repaired less effectively. In vitro and in vivo cell models were established to investigate the importance of aging as a risk factor in the development of cardiovascular disease. Cultured endothelial cells derived from rat aortic explants were identified by their characteristic cobblestone morphology, immunofluorescent staining for Factor VIII and high levels of activity of angiotensin-converting enzyme. These endothelial cells exhibited an age-associated decline in their patterns of growth and migration. In parallel with these in vitro studies, an in vivo model for examining endothelial cell growth and regeneration was developed, utilizing an inter-arterial balloon catheter as a means of producing mechanical injury. Aortic tissue from uninjured senescent rats showed increased levels of expression of mRNA for TGF-beta and fibronectin compared to the corresponding message levels in young adult rats. The relationship between hypertension and aging was investigated using freshly-isolated, single smooth muscle cells from the rat tail artery. Baseline functional studies showed an immediate rise in intracellular Ca 2+ and delayed contractile response following administration of norepinephrine or depolarizing levels of K+. The Ca 2+ signal exhibited both phasic and tonic components and was correlated with changes in cell length and agonist concentration. These results demonstrate the feasibility of using single endothelial and smooth muscle cells to study the role of aging in vascular disease.